The present invention relates to the field of blades, particularly fan blades, intended for turbojets, particularly of the aeronautical type.
The turbojet conventionally comprises a compressor, a combustion chamber and a turbine. The purpose of the compressor is to increase the pressure of the air supplied to the combustion chamber. The purpose of the turbine is to drive the rotation of the compressor by tapping off some of the pressure energy of the hot gases leaving the combustion chamber and converting it into mechanical energy.
A turbojet may be of the “bypass” type, that is to say one through which two air flows pass, a primary flow and a secondary flow. The primary flow is produced by elements that make up a single flow turbojet to which one or more additional turbines are added in order to drive a compression stage, the fan. This fan is equipped with large-sized blades, the fan blades, which produce the secondary flow. The fan slightly increases the pressure of the gases passing through it but, because its diameter is large, the energy produced for thrust is high.
One well-known example of a turbojet with a fan, also known as a turbofan is the CFM56 fitted to numerous airplanes across the world for a number of decades now. The successive series of the CFM56 have seen a gradual decrease in the number of fan blades.
Decreasing the number of fan blades on a turbojet is advantageous insofar as it allows a significant reduction in the mass of the turbojet and a reduction in procurement and maintenance costs. This reduction in the number of blades must not, however, be made at the expense of turbojet performance. Preferably, an increase in the chord length of the blades should be avoided in order to limit the size, and thus the mass, of the turbojet.
Gradually reducing the number of blades involves increasing the relative pitch better known in English as the “pitch to chord ratio” and, for the same chord length, increasing the inter-blade distance, that is to say the distance separating two consecutive blades.
The pitch to chord radio is defined as the ratio s/C where:                s represents the inter-blade pitch (s=2πR/N) N being number of blades on a bladed disk, and        C represents the chord of the profile at the radius R considered over the height of a blade, the chord C representing the length of the segment connecting the leading edge to the trailing edge of a blade.        